Various rubber compounds are formulated using a variety of different additive agents to cure the rubber and control and modify various properties of the rubber depending upon the desired end use. In formulating the compositions, these additives are selected for their usefulness o effect on modifying or providing the desired properties so as to achieve an end product which has the best attainable balance of properties depending upon the intended use of the rubber composition. Unfortunately, any particular addition agent might well and, in fact, usually does affect more than one of the properties of the final composition. Thus, while any particular additive may improve or provide a desired modification of one property, the same ingredient may degrade or undesirably affect one or more of the other properties. Thus, it is usually necessary to add a variety of different ingredients not only to change or modify the desired properties, but also to counteract the undesirable modifications or changes which are wrought or affected by other additives. Hence, in many rubber compositions there are a large number of ingredients that are added in controlled proportions in an attempt to optimize the properties for the desired end product.
For example, in rubber compositions which are for a tire belt skim, often referred to as belt skim or cord reinforced rubber composites, or steel cord-rubber composites, certain characteristics are required. The performance of these steel cord-rubber composites as used in tires, conveyor belts, etc. is determined by the properties of the individual components (i.e. steel cord and rubber individually), as well as by the adhesion between the components. To obtain good adhesion between steel cords and rubber, the steel cords used in tires or reinforced composite articles are conventionally plated with brass which is an alloy of copper and zinc. The adhesion bond between brass and rubber is formed in situ during or concurrent with the vulcanization (curing) of the rubber with sulfur. Thus, sulfur in the rubber compound has a dual purpose; i.e. it vulcanizes or crosslinks the rubber, and also promotes adhesion of the rubber to the brass plated steel cords.
The bond between brass and rubber is believed to involve the formation of a thin layer of copper sulfide. The exact stoichiometry of this copper sulfide is not known completely; however, the formation of this bond and its durability is of the utmost importance in performance and durability. The formulation or composition of the rubber in a belt skim application is therefore very critical because of several requirements that must be fulfilled, namely (1) good processing characteristics; (2) the resulting rubber must be very resistant to fatigue crack propagation ("tough" rubber); (3) the resulting adhesive bond between brass and rubber must be strong and durable, including resistance to moisture and corrosion actions in general.
Good processing characteristics refer to a rubber stock of moderate viscosity and a good balance between scorch time (t.sub.s2) and cure times (t.sub.25, t.sub.90, t.sub.100). A very short scorch time is normally not good because it does not give enough processing time for forming or shaping operations (e.g. time for the rubber to flow during calendaring, extrusion, and molding). On the other hand, a scorch time that is too long is undesirable since the total cure cycle will be prolonged and therefore decrease productivity. Similarly, too long a cure time will require excessive residence time of the tire in the mold and also result in too long an overall cure time to be commercially effective.
However, as stated above, in selecting the desired addition agents to provide these desired properties, while any given agent may have a beneficial effect on one of these properties, it may have a serious detrimental or deleterious effect on one or more of the other properties. For example, the addition of triethanolamine to a belt skim containing silica may have some beneficial effect by reducing cure time, but it has a very serious detrimental effect on the adhesion of the belt skim composition to the plated steel wires. This is just one example of many of how a particular additive may favorably or beneficial change one property and at the same time adversely affect another property, even to the extent that such an addition is commercially useless.
There have been many different prior art proposals for various combinations o additives to various types of rubber compounds.
U.S. Pat. No. 4,192,790 assigned to Uniroyal discloses the use of various additives for altering certain characteristics of different types of rubber compounds. This patent discloses, for example, the use of both peroxide and sulfur type curing agents, as well as the addition of silica, carbon black, and basic zinc methacrylate to achieve certain results. However, this patent does not disclose specifically a sulfur cured natural or diene rubber which includes high amounts of both silica and carbon black, as well as a zinc methacrylate and does not disclose a zinc dimethacrylate at all.
U.S. Pat. No. 3,823,122 assigned to Nalco Chemical Company discloses a sulfur cured styrene butadiene rubber to which carbon black and zinc methacrylate are added. It does not disclose the use of silica in this composition.
UK Patent No. 1,364,138 assigned to Acushnet Company discloses a polybutadiene rubber which has silica and zinc oxide and methacrylic acid added thereto. It does not suggest the addition of carbon black nor is this a sulfur cured product.
U.S. Pat. No. 4,929,684, assigned to Bridgestone/Firestone Inc., discloses a natural rubber and styrene butadiene rubber having carbon black and zinc oxide and stearic acid, as well as zinc dimethacrylate added thereto. It does not suggest the addition of silica.
U.S. Pat. No. 4,529,770, assigned to Firestone Tire and Rubber, discloses a peroxide cured rubber as opposed to a sulfur cured rubber which includes silica or carbon black and zinc dimethacrylate.
U.S. Pat. No. 4,495,326 assigned to Firestone Tire and Rubber Company, Inc., discloses a sulfur curable rubber including carbon black and zinc dimethacrylate. It does not teach the use of silica.
U.S. Pat. No. 4,056,269 discloses a golf ball made of a peroxide cured rubber which includes the addition of zinc oxide and methacrylic acid and zinc methacrylate. Various fillers are suggested including silica and carbon black but no specific indication is given of a sulfur cured rubber compound with carbon black and silica as well as zinc dimethacrylate.
U.S. Pat. No. 4,611,810 assigned to Toyo Denka Kogyo Co. Ltd. and Kamatari Co. Ltd. also discloses a golf ball formulation which is preferably peroxide cured, but contains a minor amount of sulfur in the formulation. No examples of silica and carbon black fillers are given.
U.S. Pat. No. 4,713,409 assigned to Firestone Tire and Rubber Co. discloses a method of producing zinc dimethacrylate and also a peroxide cured rubber with dimethacrylate added thereto.
U.S. Pat. No. 4,441,946 assigned to General Tire and Rubber Company discloses a sulfur cured natural rubber and styrene butadiene rubber mix which includes both carbon black and silica but does not disclose the use of zinc methacrylate.
A brochure from Sartomer Co. discloses a silica filled sulfur cured natural rubber compound with zinc dimethacrylate, but does not disclose carbon black.
None of the above mentioned references teach or suggest a sulfur cured natural or diene rubber having specifically both silica and carbon black and zinc dimethacrylate.